为什么我在使用 fgets 时必须输入 3 次 EOF? [英] Why do i have to input EOF 3 times when using fgets?

问题描述

所以基本上我想将我写入标准输入(包括换行符)的所有内容复制到字符串以用于哈希目的.我设法做到了这一点，并编写了一些小代码来表示我的问题.

So basically I want to copy everything i write to stdin (including newline char) to string for hash purposes. I managed to accomplish that and made small code to represent my problem.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define BUFFERSIZE 10000

int main()
{
char *myStr = calloc(1,1);
char buffer[BUFFERSIZE];

while( fgets(buffer, BUFFERSIZE , stdin) != NULL ){
  myStr = realloc(myStr, strlen(myStr)+1+strlen(buffer) );
  strcat( myStr, buffer );
}
printf("
%s
",myStr);

}

当我输入一些文本然后按 ENTER 并在我调用 EOF 之后一切正常.

everything works when I enter some text then press ENTER and after I call EOF.

但是当我启动程序输入a"然后我尝试调用EOF(使用Ctrl Z + ⏎(Windows cmd提示符)，Ctrl D (Linux)) 我必须做三遍才能让程序真正打破循环.我期待最多 2 次.

But when I start program enter "a" then I try to call EOF (using Ctrl Z + ⏎ (Windows cmd prompt), Ctrl D (Linux)) I have to do it three times for program to actually break the loop. I was expecting maximum of 2 times.

有人可以解释一下使用 EOF、stdin 和 fgets 是如何工作的吗?还是我应该使用其他东西(例如 getline)?很抱歉，如果我不清楚我的问题，请提出任何您需要的问题.

Can someone explain how using EOF, stdin and fgets works? Or should I use something else (for example getline)? I am sorry if I am not clear about my problem, just ask anything you need.

谢谢.

推荐答案

首先，^Z 或 ^D 是控制字符，它们对您正在使用的终端具有意义，有时意味着用于终端发出文件结束条件的信号.

First of all, ^Z or ^D are control characters that mean something to the terminal you are using, and sometimes that means for the terminal to signal end-of-file condition.

无论如何，在输入文本后，终端会处理您的三个按键以执行以下操作:

Anyway, your three keypresses are processed by the terminal to take the following actions, after entering text:

1. 刷新输入(即将已经输入的字符从终端发送到程序 - 默认情况下这不会发生，因为终端使用行缓冲)
2. 设置文件结束条件
3. 再次设置文件结束条件

在你的程序里面对应:

1. 什么也没发生:即使接收到 a，fgets 也会一直读取直到文件结束或换行
2. fgets 由于文件结束而完成.但是它不会返回 NULL 因为字符被读取，"a" 是特定的.
3. fgets 因为文件结束而完成，并返回 NULL 因为没有读取字符.

1. Nothing happens: even though a is received, fgets keeps reading until end-of-file or newline
2. fgets completes because of end-of file. However it does not return NULL because characters were read, "a" to be specific.
3. fgets completes because of end-of-file, and returns NULL because there were no characters read.

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